Piezoelectric actuator with an outer electrode that is adapted for thermal expansion

ABSTRACT

A piezoelectric actuator, for example for actuating a mechanical component, is proposed, which has a multilayered structure of piezoelectric layers and internal electrodes ( 2, 3 ) disposed between them. As a conductive surface, a first external electrode ( 6 ) is affixed to a respective side surface and is contacted by the respective internal electrodes ( 2, 3 ), and a second net-like or woven cloth-like external electrode ( 7 ) is disposed on the first external electrode ( 6 ). In the regions of the piezoelectric layers, which have respective internal electrodes ( 2, 3 ) that are respectively contacted on opposite sides from one another, there is a passive zone ( 10 ) without an internal electrode layer. The ceramic piezoelectric material—at least in the passive zone ( 10 )—and the material of the second external electrode ( 7 ) have almost the same thermal expansion coefficient, by means of which the influence of mechanical stresses in the piezoelectric actuator can be reduced.

PRIOR ART

The invention relates to a piezoelectric actuator, for example foractuating a mechanical component such as a valve or the like, accordingto the features contained in the preamble to the main claim.

It is generally known that using the so-called piezoelectric effect, apiezoelectric element can be constructed out of a material with asuitable crystalline structure. The application of an externalelectrical voltage causes a mechanical reaction of the piezoelectricelement which, depending on the crystalline structure and the region towhich the electrical voltage is applied, produces a compression ortension in a predictable direction. This piezoelectric actuator can beconstructed of a number of layers (multilayered actuators), wherein theelectrodes via which the electrical voltage is applied are respectivelydisposed between the layers.

Piezoelectric actuators of this kind can be provided, for example, todrive on-off valves in fuel injection systems in motor vehicles. Duringoperation of the piezoelectric actuator here, particular care must betaken that mechanical stresses in the layer structure do not also causeundesirable fractures to form in the vicinity of the external connectingelectrodes. Since the internal electrodes, which are each contacted onone side, are integrated into the layer structure in comb fashion, theelectrodes disposed one after another in the direction of the layerstructure must be respectively contacted in an alternating fashion onopposite sides from one another. When the piezoelectric actuator isactuated, i.e. when a voltage is applied between the internal electrodeson opposite sides in the layer structure, different mechanical forcesare produced in the vicinity of the internal electrodes as well as inthe vicinity of the contacts on the external electrodes, which can leadto mechanical stresses and therefore to fractures in the externalelectrodes.

ADVANTAGES OF THE INVENTION

The piezoelectric actuator described at the beginning, which can beused, for example, to actuate a mechanical component, is advantageouslymodified by virtue of the fact that with a multilayered structure ofceramic piezoelectric layers with internal electrodes disposed betweenthem, as a first external electrode, a conductive surface is affixed toa respective side surface and is contacted by the respective internalelectrodes, and a second net-like, woven cloth like, or wavy externalelectrode is disposed on the first. The second external electrode iscontacted at least at points by the first external electrode andexpandable regions are disposed between the contacts.

In the regions between two piezoelectric layers, which have a respectiveinternal electrode that is respectively contacted on the opposite sidesfrom the other, there is a passive zone without an internal electrodelayer, wherein according to the invention, the ceramic piezoelectricmaterial—at least in the passive zone —and the material of the secondexternal electrode have virtually equal thermal expansion coefficients.

The first external electrode can be a thin layer, for example a few μmthick, made of Ni, Ni+Cu, or Ni+PbSn, which adheres directly to thesurface of the piezoelectric actuator; the second external electrodehere is disposed so as to protect against lateral fractures which caninterrupt the current conduction in the external electrode. The lateralfractures can occur in the first external electrode due to delaminationin the internal electrodes as a result of tensile stresses in thepiezoelectric layers in the passive zone. Through the disposition of thenet-like second external electrode, the lateral fractures are stoppedand the current conduction, which may have been interrupted in the firstexternal electrode, is bridged.

In addition, when there are rapid temperature changes, for example from−40° C. to +160° C. in the piezoelectric actuator, there is also thepossibility of separation of the first external electrode from thepiezoelectric ceramic due to excessive shearing stresses when there istoo great a difference between the thermal expansion coefficients of thepiezoelectric ceramic material and the material of the externalelectrodes. In particular, due to its greater thickness (approx. 100μm), the second external electrode would then exert high shearing forcesin comparison to the first layer (approx 5 μm thick).

In a particularly advantageous manner, the mechanical stresses in thepiezoelectric actuator can be reduced if the ceramic piezoelectriclayers and the second external electrodes have virtually equal thermalexpansion coefficients of approx. 1*10⁻⁶*1/K to 10*10⁻⁶*1/K. The ceramicpiezoelectric layers can be made of lead zirconate titanate and thesecond external electrodes can be made of iron-nickel alloys, e.g.Invar. In order to improve ability of this material to be soldered, itcan first be coated with a thin (e.g. 5-10 μm) layer of copper. A Sn-Pbsolder layer can then be deposited in order to solder the secondexternal electrode onto the first.

These and other features of preferred modifications of the inventionensue not only from the claims but also from the specification and thedrawings, wherein the individual features can be respectively realizedsingly or multiply in the form of sub-combinations in the embodiment ofthe invention and in other areas and can represent advantageous andintrinsically patentable embodiments which are claimed herein.

DRAWINGS

An exemplary embodiment of the piezoelectric actuator according to theinvention will be explained in conjunction with the drawings.

FIG. 1 is a section through a piezoelectric actuator that has amultilayered structure of internal electrodes and piezoelectric ceramiclayers and has a net-like external electrode deposited on a first flatexternal electrode;

FIG. 2 is a top view of the net-like external electrode according toFIG. 1; and

FIG. 3 is a detailed section through the layer structure and theexternal electrodes in passive zones of the piezoelectric layers, withthe fractures that are produced there due to mechanical shearingstresses and lateral stresses that occur.

DESCRIPTION OF THE EXEMPLARY EMBODIMENTS

FIG. 1 shows a piezoelectric actuator 1 which is comprised in anintrinsically known manner of piezoelectric foils of a ceramic materialwith a suitable crystalline structure so that that using the so-calledpiezoelectric effect, the application of an external electrical voltageto internal electrodes 2 and 3 causes a mechanical reaction of thepiezoelectric actuator 1. FIG. 2 shows a side view of this device.

The internal electrodes 2 and 3 are contacted by a first, flat externalelectrode 6 which is in turn contacted by a second net-like externalelectrode 7 by means of point contacts 8, for example by means ofsolders or welds. The first external electrode 6 can be a thin layer,for example a few μm thick, made of Ni, Ni+Cu, or Ni+PbSn, which adheresdirectly to the surface of the piezoelectric actuator.

The detailed sectional view according to FIG. 3 clearly shows the layerstructure and the laterally adjoining external electrodes 6 and 7according to FIGS. 1 and 2. A passive zone 10 is indicated here in whichthe electrical field according to the arrows 11 is not sharplypronounced, as a result of which the expansion of the piezoelectricactuator 1 does not occur freely, but is forced by means of tensilestress in the ceramic material of the piezoelectric actuator 1. As aresult, delaminations 12 can occur, which are indicated here at severalelectrodes 3. The delamination 12 can turn into a fracture 13 in thefirst external electrode 6. The fracture 13, however, is stopped by thenet-like second external electrode 7 and is conductively bridged.

Furthermore, shearing fractures 14 are indicated in FIG. 3, which canoccur between the ceramic material of the piezoelectric layers and thefirst external electrode 6 due to differing thermal expansion of thepiezoelectric ceramic and the second external electrode 7. Therelatively thin first external electrode 6 does not have any greatinfluence here.

What is claimed is:
 1. A piezoelectric actuator, comprising: amultilayered structure of ceramic piezoelectric layers with internalelectrodes (2, 3) disposed between said layers; external electrodes (6,7), said external electrodes arranged to alternatingly and laterallycontact said internal electrodes (2, 3), wherein said externalelectrodes supply an external voltage, wherein a first externalelectrode (6) is a conductive surface affixed to a side surface of saidmultilayered structure and is contacted by a respective one of saidinternal electrodes (2, 3), and wherein a second external electrode (7)comprises a net-like, woven cloth-like, material, said second externalelectrode disposed on the first external electrode (6), wherein thesecond external electrode is contacted at least at contact points by thefirst external electrode (6), wherein expandable regions are disposedbetween the contact points; and at least one passive zone (10) in saidmultilayered structure having no internal electrode, and wherein theceramic piezoelectric layers—at least in the at least one passive zone(10)—and the material of the second external electrode (7) have almostthe same thermal expansion coefficient as one another.
 2. Thepiezoelectric actuator according to claim 1, wherein the ceramicpiezoelectric layers and the material of the second external electrode(7) each have a thermal expansion coefficient of approximately1*10⁻⁶*1/K to 10*10⁻⁶*1/K.
 3. The piezoelectric actuator according toclaim 2, wherein the ceramic piezoelectric layers are made of leadzirconate titanate.
 4. The piezoelectric actuator according to claim 2or 3, wherein the second external electrode (7) is made of aniron-nickel alloy.